Letter to the Editor

The K20 survey^*

I. Disentangling old and dusty star-forming galaxies in the ERO population

¹ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Dipartimento di Astronomia, Università di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy
³ Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Bologna, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching, Germany
⁵ Racah Institute for Physics, The Hebrew University, Jerusalem, 91904, Israel
⁶ Osservatorio Astronomico di Roma, via Dell'Osservatorio 2, Monteporzio, Italy
⁷ Osservatorio Astronomico di Brera, via E. Bianchi 46, Merate, Italy
⁸ Dipartimento di Astronomia, Università “La Sapienza”, Roma, Italy
⁹ ST, European Coordinating Facility, Karl-Schwarzschild-Str. 2, 85748, Garching, Germany

Corresponding author: A. Cimatti, cimatti@arcetri.astro.it

Received: 12 October 2001
Accepted: 28 November 2001

Abstract

We present the results of VLT optical spectroscopy of a complete sample of 78 EROs with over a field of 52 arcmin². About 70% of the 45 EROs with have been spectroscopically identified with old passively evolving and dusty star-forming galaxies at . The two classes are about equally populated and for each of them we present and discuss the average spectrum. From the old ERO average spectrum and for we derive a minimum age of ~3 Gyr, corresponding to a formation redshift of z_f  ≳ 2.4. PLE models with such formation redshifts well reproduce the density of old EROs (consistent with being passively evolving ellipticals), whereas the predictions of the current hierarchical merging models are lower than the observed densities by large factors (up to an order of magnitude). From the average spectrum of the star-forming EROs we estimate a substantial dust extinction with   ≳ 0.5. The star formation rates, corrected for the average reddening, suggest a significant contribution from EROs to the cosmic star-formation density at .